1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar electrophotographic image forming apparatus, particularly an image forming apparatus of the type including a plurality of image carriers arranged along an image transfer belt and an image transferring device configured to transfer toner images of different colors from the image carriers to a sheet being conveyed by an image transfer belt or by way of the image transfer belt by applying a bias to the belt. More particularly, the present invention relates to a bias applying method for an image forming apparatus of the type described and a device for the same.
2. Description of the Background Art
Today, a color copier, color printer or similar color image forming apparatus is spreading and includes either a single photoconductive drum or a plurality of photoconductive drums arranged in a tandem configuration. In the color image forming apparatus including a single drum, a plurality of developing units are arranged around the drum, and each forms a toner image on the drum in a particular color. Toner images so formed on the drums are transferred to a sheet one above the other, completing a full-color image. In the tandem color image forming apparatus, the drums or image carriers are arranged along the surface of a transfer belt. Toner images formed on the drums in respective colors are transferred to a sheet, OHP (Over Head Projector) sheet or similar recording medium either directly or indirectly with a bias being applied to the transfer belt.
The color image forming apparatus with a single drum is small size and low cost. However, to form a full-color image, the apparatus has to repeat image formation a plurality of times (usually four times) with the drum, resulting in a long image forming time that obstructs high-speed image formation. By contrast, the tandem image forming apparatus can form a full-color image with a plurality of (usually four) drums and therefore at high speed although it is bulky and high cost.
The tandem color image forming apparatus uses either one of a direct image transfer system and an indirect image transfer system. In the direct image transfer system, intermediate image transferring devices corresponding one-to-one to the drums transfer toner images of different colors from the drums to a sheet being conveyed by a conveying belt one above the other. In the indirect image transfer system, primary image transferring devices transfer toners of different colors from the drums to an intermediate image transfer belt one above the other. Subsequently, a secondary image transferring device transfers the resulting full-color image from the intermediate image transfer belt to a sheet.
A problem with the direct image transfer system is that a sheet feeder and a fixing unit should be respectively positioned upstream and downstream of the plurality of drums arranged along the conveying belt, increasing the size of the apparatus body in the direction of sheet conveyance. By contrast, the indirect image transfer system allows the secondary image transfer devices to be relatively freely laid out, so that the sheet feeder and fixing unit can be arranged one above the other below the drums. This successfully reduces the overall size of the apparatus body.
Another problem with the direct image transfer system is that when the fixing unit is positioned near the most downstream drum in order to reduce the size in the direction of sheet conveyance, a sufficient path for a sheet to bend cannot be provided between the drum and the fixing unit. Consequently, the fixing unit is apt to adversely influence image formation effected at the upstream side due to an impact ascribable to the leading edge of a sheet entering the fixing unit or a difference between the speed of the sheet passing the fixing unit and the speed of the conveying belt. The indirect image transfer system guarantees a sufficient path for a sheet to bent and is therefore free from such a problem.
As for a modern color image forming apparatus, there is an increasing demand for full-color image formation as rapid as monochromatic image formation. In this respect, the tandem color image forming apparatus, particularly one using the indirect image transfer system, is attracting increasing attention.
It has been reported in relation to the tandem, indirect image transfer type color image forming apparatus that image degradation ascribable to, e.g., the scattering of toner can be improved if the intermediate image transfer belt has an outer surface layer provided with high resistance. However, when the intermediate image transfer belt has a single layer with high resistance, it is difficult to set a position where an adequate bias for primary image transfer should be applied to the belt for the transfer of a toner image from the drum to the belt. Even a shift of the above position by several millimeters results in defective image transfer. More specifically, the bias for primary image transfer is intended to form an electric field in a gap between the drum and the intermediate image transfer belt for thereby transferring a toner image from the drum to the intermediate image transfer belt. Should the electric field not lie in an adequate range, a toner image transferred to the intermediate image transfer belt would be irregular.
In light of the above, the inner surface or reverse surface of the intermediate image transfer belt to which the bias is to be applied may be provided with medium resistance. Medium resistance equalizes potentials around the portion of the intermediate image transfer belt to which the bias is applied, thereby broadening the range of the adequate position where the bias should be applied.
However, the intermediate image transfer belt with the inner surface having medium resistance has a problem that the bias applied to the expected portion of the belt for primary image transfer leaks. Further, the inner surface layer of the intermediate image transfer belt is generally formed of a material with carbon black or similar conduction agent dispersed therein or an ion-conductive material. However, the material with a conductive agent dispersed therein has a disadvantage in that the dispersion of the agent is irregular due to production reasons. The ion-conductive material has a disadvantage that resistance thereof is apt to vary due to, e.g., the varying environment, e.g., temperature and humidity. It is therefore necessary to adequately control the bias to be applied to the intermediate image transfer belt.
Some different schemes customarily used to control the bias for the intermediate image transfer belt will be described hereinafter. A first scheme is constant voltage control. When a constant voltage bias is applied to the intermediate image transfer belt, the leak of a current mentioned above does not occur. However, when the charge potential or the resistance of the high-resistance layer forming the outer surface of the belt is irregular, the constant voltage control cannot maintain a current to flow toward the drum constant. More specifically, as for the charging of the high-resistance layer, the potential condition is necessarily effected by history and therefore results in the aggravation of noise.
A second scheme is providing the medium resistance layer on the inner surface of the intermediate image transfer belt with relatively high resistance close to the upper limit to thereby reduce the mutual influence of the primary image transfer positions as far as possible. However, the prerequisite with this scheme is that the resistances of the materials constituting the belt be strictly standardized, resulting in low yield and high cost.
A third scheme uses a differential constant current. This scheme measures a leak current leaking around the intermediate image transfer belt and adding the leak current to the bias beforehand to thereby indirectly maintain the current to flow toward the drum constant. The differential constant current scheme is customary with a belt transfer type monochromatic machine or an intermediate image transferring device included in a revolver type (non-tandem type) machine.
The third scheme, however, cannot be applied to the tandem, intermediate image transfer type image forming apparatus for the following reason. In this type of image forming apparatus, currents to flow at nearby primary image transfer positions noticeably influence each other. Moreover, which power source output should be controlled is not known. More specifically, such an image forming apparatus includes, e.g., four power supplies each for applying a bias to bias applying means located at a particular image transfer position. Therefore, even when a leak current is sensed at both ends of the intermediate image transfer belt, a portion where the current is leaking cannot be located.
As for the tandem, intermediate image transfer type image forming apparatus, there has been proposed a method that directly connects an ammeter between nearby bias applying means in order to measure their relation. For example, an ammeter using an optical fiber output is positioned between nearby high voltages so as to perform calculation with the output of the ammeter. This kind of configuration is available on the market as a current metering unit highly resistive to noise for use in factories. However, a plurality of such current metering units installed in the image forming apparatus would result in a prohibitive cost.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2000-137366.
It is an object of the present invention to provide a bias applying method capable of accurately estimating the DC leak current of a bias applied to an image transfer belt to thereby maintain a differential current between a plurality of power supplies constant, a device for the same, and an image forming apparatus including the device. A bias applying method of the present invention is applicable to a bias applying device configured to form, at each of image transfer positions where a plurality of image carriers and an image transfer belt moving in contact with the surfaces of the image carriers, an electric field for transferring a toner image formed on each image carrier to a transfer medium by applying a bias to the image transfer belt. The bias applying device includes a plurality of bias applying means each for applying a bias to the image transfer belt at the respective image transfer position. A plurality of high-potential power supply sections each are connected to one of the bias applying means for applying a bias, which consists of a DC component and a particular AC component superposed on the DC component, to the respective bias applying means. A plurality of sensing sections each are connected one of the bias applying means for sensing the AC component of the bias of the respective bias applying means. A central processing unit controls the high-tension power supply sections and sensing sections. The bias applying method detects the AC component of a second high-tension power supply section, which is detected at the output of a first high-tension power supply section, determines an AC resistance between the first and second high-tension power supply sections on the basis of the absolute value of the AC component detected, estimates the leak current of a DC component by referencing a table listing a correlation between AC resistances and DC resistances and prepared beforehand, and adds the leak current to a set DC value assigned to the first high-tension power supply section to thereby correct the bias.
A bias applying device for practicing the above method and an image forming apparatus including the same are also disclosed.